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This was originally a medium, but I did a lot of thinking around this, and there are some tricky parts to making this feature work. I assess that it is a large task. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f0a943faef14c2772d5b304e888098ac7f35fa99

commit f0a943faef14c2772d5b304e888098ac7f35fa99
Author: edchin <edchin@chromium.org>
Date: Wed May 02 21:05:07 2018

[ios] Enable undo close all tabs

This CL enables undo close all tabs in the tab grid by serializing
the current window. The undo operation deserializes the saved closed
window.

Current known issue is that snapshots are erased when WebStates are
closed. Another CL will deal with keeping snapshots around long enough
for undo operations.

Bug:  804567 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I12cf8c11d1e067a7de1b9a6ce44d207810619eb8
Reviewed-on: https://chromium-review.googlesource.com/1036257
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555520}
[modify] https://crrev.com/f0a943faef14c2772d5b304e888098ac7f35fa99/ios/chrome/browser/ui/tab_grid/grid/grid_commands.h
[modify] https://crrev.com/f0a943faef14c2772d5b304e888098ac7f35fa99/ios/chrome/browser/ui/tab_grid/grid/grid_view_controller.mm
[modify] https://crrev.com/f0a943faef14c2772d5b304e888098ac7f35fa99/ios/chrome/browser/ui/tab_grid/tab_grid_mediator.mm
[modify] https://crrev.com/f0a943faef14c2772d5b304e888098ac7f35fa99/ios/chrome/browser/ui/tab_grid/tab_grid_mediator_unittest.mm
[modify] https://crrev.com/f0a943faef14c2772d5b304e888098ac7f35fa99/ios/chrome/browser/ui/tab_grid/tab_grid_view_controller.mm
[modify] https://crrev.com/f0a943faef14c2772d5b304e888098ac7f35fa99/ios/chrome/browser/web_state_list/web_state_list_serialization_unittest.mm
[modify] https://crrev.com/f0a943faef14c2772d5b304e888098ac7f35fa99/ios/web/public/test/fakes/test_web_state.mm

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/45ab3fe6bfcec219fad4e0f48bdd03c03c31281c

commit 45ab3fe6bfcec219fad4e0f48bdd03c03c31281c
Author: edchin <edchin@chromium.org>
Date: Fri May 04 19:56:56 2018

[ios] Restore snapshot for undo close all tabs

When closing tabs, an observer immediately deletes snapshots from the
snapshot cache. This prevents snapshots from reappearing in an undo
close all tabs operation.

This CL enables snapshot restoration on undo close all tabs by extending
the life of a snapshot when closed with a "close all" operation. Note
this is only extended for regular tabs, not incognito tabs.

The extended life is accomplished by marking tabs for deletion during
a close all operation, rather than immediately deleting the image
from the cache. Later, the marked images are either purged from the
cache, or the marked list is cleared (therefore leaving the images in
the cache).

Bug:  804567 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I4710bf46d3ebd4ad347563e5f6a6208a68a58209
Reviewed-on: https://chromium-review.googlesource.com/1041545
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556162}
[modify] https://crrev.com/45ab3fe6bfcec219fad4e0f48bdd03c03c31281c/ios/chrome/browser/snapshots/snapshot_cache.h
[modify] https://crrev.com/45ab3fe6bfcec219fad4e0f48bdd03c03c31281c/ios/chrome/browser/snapshots/snapshot_cache.mm
[modify] https://crrev.com/45ab3fe6bfcec219fad4e0f48bdd03c03c31281c/ios/chrome/browser/snapshots/snapshot_cache_unittest.mm
[modify] https://crrev.com/45ab3fe6bfcec219fad4e0f48bdd03c03c31281c/ios/chrome/browser/ui/tab_grid/tab_grid_mediator.mm

The NextAction date has arrived: 2018-05-09

Verified in 68.0.3431.0 Canary, iPhone X iOS 11.4 beta 4, iPhone 6 iOS 10.3.3
Undo for Close All is Enabled.